Multi-color tunable Ce3+–Mn2+ cooperative Y7O6F9 vernier phosphors

Multi-color tunable Ce3+–Mn2+ cooperative Y7O6F9 vernier phosphors

Journal of Alloys and Compounds 673 (2016) 1e7 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://ww...

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Journal of Alloys and Compounds 673 (2016) 1e7

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Multi-color tunable Ce3þeMn2þ cooperative Y7O6F9 vernier phosphors Wonseok Yang, Sung-Hoon Kim, Sangmoon Park* Center for Green Fusion Technology, Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2015 Received in revised form 15 February 2016 Accepted 28 February 2016 Available online 3 March 2016

Ce3þeMn2þedoped Y7O6F9 vernier phosphors composed of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.005e0.1, q ¼ 0e0.1) were prepared using a fluxeassisted solid-state reaction. The X-ray diffraction patterns of the resultant phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Ce3þ-activated yttriumeoxyfluoride phosphors were clearly monitored with critical emission quenching as a function of Ce3þ content in the Y7(1-p-2q/3)Ce7pMn14q/3O6F9. After doping the Y7O6F9 structure with Ce3þ and Mn2þ activators, intense blue and green/orange emission lights were observed in the PL spectra under near-ultraviolet (NUV) excitation. The dependence of the luminescent intensity of the Mn2þ coedoped (q ¼ 0, 0.01, 0.05, 0.1) host lattices on Ce3þ content (p ¼ 0.05, 0.1) was also studied. Co-doping Mn2þ into the Ce3þedoped host structure enabled high energy transfer from Ce3þ to Mn2þ; this energy transfer mechanism is discussed. Multi-color tunable blue, white, yellow, and green emission lights due to the Ce3þ and Mn2þ emitters were observed at room temperature. With these phosphors, the desired CIE values including emissions throughout multiecolor regions of the spectra were achieved. © 2016 Elsevier B.V. All rights reserved.

Keywords: Vernier phase X-ray diffraction Phosphors Photoluminescence Energy transfer

1. Introduction Since 1997, phosphor-converted white lighteemitting diodes (LEDs) based on a blue LED chip combined with a yellow Y3Al5O12:Ce phosphor have been efficiently used as light sources, that currently replaces incandescent and fluorescent bulbs. Their applications are also widely spreading out to fields such as agriculture, visible light communication, and human health [1e3]. Although yellowephosphor/blueechip combined whiteeLEDs have advantages of low cost and easy fabrication, they also exhibit weaknesses such as low color render indices (CRI) and chromatic stability. While redegreeneblue (RGB) phosphors/UVechip LEDs can exhibit high CRI and chromatic stability, the complex blending of various RGB phosphors is a noticeable disadvantage [3]. Singleephase phosphoreconverted phosphors/UVechip LED exhibit excellent CRI values and color stability with color-tunable and easy fabrication by harmonizing activators and sensitizers via an energy-transfer mechanism [3,4]. Rareeearth Ce3þ and Eu2þ ions are the most commonly used activators as well as sensitizers owing

* Corresponding author. E-mail address: [email protected] (S. Park). http://dx.doi.org/10.1016/j.jallcom.2016.02.225 0925-8388/© 2016 Elsevier B.V. All rights reserved.

to their wide emission bands resulting from ded transitions with efficient absorption. Owing to the paucity of red light in the spectral emission, Mn2þ ions attributed to the ded transition (4T1 / 6A1) can be extensively used with the Ce3þ and Eu2þ ions for singleephase color-tunable phosphors. The ded transition of Mn2þ ions is difficult to observe because the corresponding electric dipole is forbidden. However, the emission of Mn2þ ions, which occurs from the green to orange/red regions, is strongly dependent on the coordination environment of the host structures. When Mn2þ ions occupy the 4e and 8ecoordinated sites in host lattices such as Zn2SiO4, Zn2GeO4, CaF2, and CeF3, green emission can usually be obtained owing to their weak crystal field, whereas orange or red emission lights are caused by strong crystal fields beside the 4e and 8efold environment of Mn2þ ions in the host lattices, e.g., ZnF2, KMgF3, Ca5(PO4)3F, CePO4 [5e7]. When multi-pole interaction is initiated by the electrostatic energy transfer between the sensitizer and the activator, the emission of Mn2þ ions becomes superior. In previous reports, singleephase coloretunable phosphors involving energy transfer between Ce3þ and Mn2þ were widely studied in various hosts such as Ca3Sc2Si3O12, Ba9Y2Si6O24, Ba2Li2Si2O7, CaSiO3, BaAl2O4, and Ba2Ca(BO3)2 [4,8e12]. The structure of Y7O6F9 shown in Fig. 1, which is called the vernier phase with space group Abm2, is known as a one-dimensional superstructure of fluorite with the

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and orange regions of the spectra were achieved. 2. Experimental Optical materials of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.005e0.1, q ¼ 0e0.1) were prepared by heating the appropriate stoichiometric amounts of Y2O3 (Alfa 99.9%), CeO2 (Alfa 99.9%), and MnO (Aldrich 99%), and NH4F (Alfa 99%) in pellets at 1000  C for 2 h under the atmosphere using 4%H2/96%Ar. The flux-assist method used in our previous report for YOF:Eu and YnOn-1Fnþ2:Eu (n ¼ 6, 7) based phosphors was utilized in previous study employed a modified molar ratio of the ½Y2O3 (Eu2O3) precursors and NH4F flux at various temperatures for the preparation of Y1-xEuxOF and Yn(1-x)EunxOn-1Fnþ2 crystal structures [14]. Phase identification was established using a Shimadzu XRD-6000 powder diffractometer (CueKa radiation) and the unit cell parameters were determined by using the Rietveld refinement program Rietica. UV spectroscopy to measure the excitation and emission spectra of the optical materials was done using spectrofluoroemeters (Sinco Fluoromate FSe2) at room temperature. 3. Results and discussion The phase of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.005e0.1, q ¼ 0e0.1) was identified by powder X-ray diffraction (XRD) analysis after Ce3þ and Mn2þ ions were substituted for Y3þ ions in the yttrium oxyfluoride host lattices. The vernier phase Y7O6F9:Eu phosphors were previously studied at high temperature using NH4F flux [14]. In this report, the preparation process included the modification of the flux molar ratio and the synthesis temperature to control the stoichiometry of the Ce3þeMn2þ doped Y7O6F9 phosphors. Fig. 2 (a) e (d) show the calculated XRD patterns of the Y2O3 (ICSD 27772), YF3 (ICSD 26595), YOF (ICSD 14282), and Y7O6F9 (ICSD 68951) structures. Fig. 1 (e) e (h) show the XRD patterns of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (e) p ¼ 0.05, q ¼ 0, (f) p ¼ 0.01, q ¼ 0.01,  (g) p ¼ 0.1, q ¼ 0.1 synthesized at 1000 C using 1:2 M ratio of Y3þ 3þ 2þ (Ce , Mn ) ions to NH4F flux. When the molar ratio of the NH4F flux was doubled, the Y2O3 precursor reacted completely, as shown in Fig. 2 (i)e(k). The mixture with a 1:2 M ratio of Y3þ (Ce3þ, Mn2þ) ions to NH4F flux, clearly showed the Y7O6F9 vernier phase, and the YF3 structure was barely observed. The unit cell of Y7O6F9 (ICSD 68951) host lattice is a ¼ 5.423(1) Å, b ¼ 38.624(6) Å, c ¼ 5.527(1) Å. When the Ce3þ ions (r ¼ 1.07 Å, CN ¼ 7 and r ¼ 1.143 Å, CN ¼ 8) were substituted in the Y sites (r ¼ 0.96 Å, CN ¼ 7 and r ¼ 1.019 Å, Fig. 1. The structure of Y7O6F9 vernier phase.

unit cell a  7b  c. The Y3þ ions are slightly displaced from the ideal fluorite sites and coordinated by four O2 and three F anions (YO4F3) and four O2 and four F anions (YO4F4) [13,14]. A single YOþ layer is sandwiched between F layers in the Y7O6F9 vernier phase. In this work, neareultraviolet (NUV)eexcitable and yttriumoxyfluoride-based optical materials composed of Y7(1-p-2q/ 3)Ce7pMn14q/3O6F9 (p ¼ 0.005e0.1, q ¼ 0e0.1) were prepared and their X-ray diffraction patterns characterized. The photoluminescence (PL) spectra, which exhibited an efficient blue emission attributed to the fed transitions of a Ce3þ emitter in the Y7O6F9, were analyzed. Moreover, the multiecolor tunable PL spectra of Ce3þ and Mn2þ coedoped Y7O6F9 phosphors under NUV excitation were monitored. The dependence of the luminescent intensity and energyetransfer mechanism of the Mn2þ coedoped (q ¼ 0e0.1) host lattices on Ce3þ content (p ¼ 0.05, 0.1) was also studied. With these phosphors, the desired CIE values including multietunable emission lights throughout the blue, white, green

Fig. 2. XRD patterns of (a) Y2O3 (ICSD 27772), (b) YF3 (ICSD 26595), (c) YOF (ICSD 14282), (d) Y7O6F9 (ICSD 68951), Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (e) p ¼ 0.05, q ¼ 0, (f) p ¼ 0.01, q ¼ 0.01, and (g) p ¼ 0.1, q ¼ 0.1.

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CN ¼ 8) of the Y7O6F9 structure, the unit cell of Y7(1-p)Ce7pO6F9 (p ¼ 0.05) was slightly expanded, a ¼ 5.416(6) Å, b ¼ 38.633(40) Å, c ¼ 5.558(11) Å. Furthermore, when the small Mn2þ ions (r ¼ 0.90 Å, CN ¼ 7 and r ¼ 0.96 Å, CN ¼ 8) were added in the Y sites of the Ce3þ-doped Y7O6F9 host lattice (Y7(1-p-2q/3)Ce7pMn14q/3O6F9, p ¼ 0.1 q ¼ 0.1), the unit cell fairly decreased in size as a ¼ 5.415(7) Å, b ¼ 38.596(56) Å, c ¼ 5.535(15) Å. Fig. 3 shows the PL spectra of Y7(1-p)Ce7pO6F9 (p ¼ 0.005e0.1) phosphors that exhibit intense blue emission. There are two main excitation peaks around 310 and 330 nm attributed to the crystal field splitting of the 5d levels of Ce3þ transitions. The maximum intensity points of the emission bands were monitored approximately 360 and 380 nm along with an increase in the molar concentrations of Ce3þ. When the Ce3þ content reached p ¼ 0.05 (~0.5 mol%) in Y7(1-p)Ce7pO6F9 phosphors, the relative luminescent intensity calculated using the integrated emission was maximized. The influence of emission peak shifts in Y7(1-p)Ce7pO6F9 phosphors with the increase of Ce3þ concentration was quite weak because of the low crystal field effect by slight expansion of the unit cell, replacing Y3þ with Ce3þ ions, as mentioned above [15]. The excitation and emission states of the Ce3þ-activated Y7O6F9eblue phosphors were shifted to a much shorter wavelength than those of the commercial YAG:Ce (Y3Al5O12:Ce) yellow phosphor, as a result of the decrease in the covalent behaviors. The relative quantum efficiency of Y7(1-p)Ce7pO6F9 (p ¼ 0.05) phosphor, which was calculated based on integrated emission, was about 50% that of YAG:Ce. Once the maximum luminescent intensity reached the Ce3þ content corresponding to p ¼ 0.05, a further increase in the Ce3þ content in the yttrium oxyfluoride phosphors led to a clear quenching of the integrated intensity of the blue emission, as shown in the inset of Fig. 3. As the concentration of Ce3þ ions increased, the distance between the activators decreased with increasing energy transfer. A decrease in the emission intensity resulted in nonradiative energy transfer between the activators due to the electric dipoleedipole interaction. The critical distance (Rc) was calculated using the following formula: Rc ¼ 2[3V/4pmcN]1/3

(1)

where V is the volume of the unit cell, N is the number of available sites for the dopant in the unit cell, pc is the critical concentration of Ce3þ, and Rc is the critical distance for energy transfer [8,16e18]. N and V are 7 and 1163.02 Å3 for the Y7O6F9 host, respectively. When p ¼ 0.05, Rc for Y7(1-p)Ce7pO6F9 was determined to be 11.7 Å. It is

Fig. 3. The excitation and emission spectra of Y7(1-p)Ce7pO6F9 (p ¼ 0.005e0.1) and the integrated intensity as a function of the content Ce3þ ions (inset).

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difficult to observe the Mn2þ ded transition because the electric dipole transition is forbidden. However, the emission corresponding to Mn2þ transitions in host lattices could be intensified through electronic dipoleequadrupole interaction by co-doping a sensitizer such as Ce3þ or Eu2þ ions [8,18]. The luminescent excitation centers of Mn2þ ions were assigned as 4A2(F), 4T1(F), 4T1(P), 4E(D), and 4 T2(D) between 250 and 400 nm, and their emissions were due to the ded transition [4T1(G) / 6A1(S)] of green and orange lights, which is consistent with previous reports [5e7]. Fig. 4(a) and (b) show the PL emission spectra and relative intensity of the Y7(1-p-3q/2)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1, q ¼ 0e0.1) phosphors. Co-doping Mn2þ into the Ce3þ-doped host structure enabled a high energy transfer from Ce3þ to Mn2þ. The Ce3þ blue emission of Y7(1-p)Ce7pO6F9 (p ¼ 0.05, 0.1) phosphor centered around 360 nm distinctly decreased when the Mn2þ content corresponded to q ¼ 0.1. In Fig. 4(c), analogous to the phosphor, the Ce3þ luminescent intensities of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors for Ce3þ content corresponding to p ¼ 0.05 increased as the Mn2þ content increased up to q ¼ 0.01. However, the maximum Mn2þ orange emission of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.1) phosphor was observed when the Mn2þ content corresponded to q ¼ 0.05. Any further increase in the Mn2þ content of the phosphors led to an apparent quenching of the relative intensity (I550nm) of the orange emission. In this Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1, q ¼ 0e0.1) structure, a transfer of energy from Ce3þ to Mn2þ occurred through the absorption of Ce3þ. Moreover, the Ce3þ and Mn2þ ions played the roles of sensitizer and activator, respectively. The energy transfer efficiency (hT) between the Ce3þ and Mn2þ ions was calculated using the formula:

hT ¼ 1 e IS/ISO

(2)

where IS and ISO are the luminescence intensities of the sensitizer in the presence and absence of an activator, respectively [8,18,19]. By using this expression, the energy transfer efficiency from Ce3þ to Mn2þ was calculated, as shown in Fig. 4(d). As the Mn2þ content in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1) phosphors increased from q ¼ 0.01 to 0.05, the efficiency at each level was extensively enhanced by up to 92 and 71%, respectively. When the Mn2þ content in the Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1) phosphors increased to q ¼ 0.1, the efficiency of the energy transfer further increased to 99 and 91%, respectively. By maintaining the Ce3þ concentration corresponding to p ¼ 0.05 in the Y7(1-p-2q/ 3)Ce7pMn14q/3O6F9 phosphors, q was varied from 0.01 to 0.1. Consequently, the efficiency of energy transfer to Mn2þ ions abruptly increased, and was maximized compared to the other energy transfers. For the Ce3þ concentration of p ¼ 0.05 and 0.1 in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors, the critical concentration from the total concentration of the Ce3þ and Mn2þ ions, at which the energy-transfer efficiency is 0.5, was much below 0.01. From equation (1), the critical distance Rc of the Y7(1-p-2q/3)Ce7pMn14q/ 3O6F9 (p ¼ 0.1, q ¼ 0.1) was calculated to be about 7.3 Å. This energy transfer between Ce3þ and Mn2þ ions could be caused by an electric multipolar interaction. This is because the critical distance, 7.3 Å, is quite longer than 3e4 Å, indicating an energy transfer through the exchange interaction mechanism. Furthermore, according to the Dexter theory, the energy transfer mechanism can be expressed by /3 the linear plots of ISO/IS versus CaMn , where CMn is concentration of Mn2þ ions, with a ¼ 6, 8, or 10, which corresponds to dipoleedipole, dipoleequadrupole, or quadrupoleequadrupole interactions, respectively. When a ¼ 6, the linear plots show the energy transfer from the Ce3þ to Mn2þ ions in the case of Y7(1-p-2q/3)Ce7pMn14q/ 2 2 3O6F9 phosphors (R ¼ 0.9916, p ¼ 0.05 and R ¼ 0.9817, p ¼ 0.1) in Fig. 4(e). Moreover, because the values of a that are determined from the better linear plots for Y7(1-p-2q/3)Ce7pMn14q/3O6F9

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Fig. 4. The emission spectra of the Y7(1-p-3q/2)Ce7pMn14q/3O6F9 phosphors (a) p ¼ 0.05, q ¼ 0e0.1, (b) p ¼ 0.1, q ¼ 0e0.1, (c) the relative intensity of the phosphors, (d) the energy /3 transfer efficiency (hT) from Ce3þ to Mn2þ in the phosphors, (e) (f) the plot of ISO/IS versus CaMn (a ¼ 6, 8).

phosphors (R2 ¼ 1, p ¼ 0.05, and R2 ¼ 0.9978, p ¼ 0.1) are close to 8 in Fig. 4(f), it appears that the dipoleequadrupole interaction is involved in the energy transfer mechanism [8,18,20,21]. Fig. 5(a) and (b) show the excitation and emission PL spectra and photos of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1, q ¼ 0.05, 0.1) phosphors. The emission spectra of the Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, q ¼ 0.05) phosphors upon excitation at 263 and 312 nm comprise of the Ce3þ blue emission centered around 380 nm and

the Mn2þ green and orange emissions centered around 530 and 555 nm, respectively. Furthermore, the emission spectra of the Y7(1p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.1, q ¼ 0.1) phosphors excited at 260 and 312 nm show green and orange lights centered around 508 and 565 nm, respectively. The green and orange emission lights caused by Ce3þeMn2þ energy transfer process upon excitation both approximately at 260 and 312 nm in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors were simultaneously monitored. The coordination

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Fig. 5. The PL spectra of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors (a) p ¼ 0.05, q ¼ 0.05 (b) p ¼ 0.1, q ¼ 0.1, (c) p ¼ 0.05, q ¼ 0e0.1, (d) p ¼ 0.1, q ¼ 0e0.1, and (e) the relative intensity of the phosphors.

environment of the luminescent center of Mn2þ ions in hosts strongly influences the emission lights. As shown in Fig. 1, the Y3þ ions are slightly displaced from the ideal fluorite sites and are coordinated by 7 with four O2 and three F anions (YO4F3) and 8 with four O2 and four F anions (YO4F4) in the structure of Y7O6F9.

The Mn2þ activators can occupy two different 7e and 8ecoordinated yttrium sites in the host lattice. As reported previously, when the Mn2þ ions occupied the 8-coordinated yttrium site in host lattices, the green emission was observed by a weak crystal field environment, whereas the luminescence center of

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7ecoordinated yttrium site in the host structure was obtained in longer wavelength showing orange or red light owing to the strong crystal field effect [5e7]. The Ce3þ blue emission of Y7(1-p)Ce7pO6F9 (p ¼ 0.05) phosphor centered around 360 nm distinctly decreased when the Mn2þ content corresponded to q ¼ 0.1, whereas the Mn2þ green emission of the phosphors centered around 508 nm were increased up to q ¼ 0.05 (Mn2þ content) and then abruptly decreased, as shown in Fig. 5(c). The Ce3þ blue emission of Y7(1p)Ce7pO6F9 (p ¼ 0.1) phosphor centered around 360 nm similarly decreased when the Mn2þ content corresponded to q ¼ 0.1, similar to the Y7(1-p)Ce7pO6F9 (p ¼ 0.05) phosphor. However, a continuous increase of in the Mn2þ green emission centered around 508 nm was observed up to q ¼ 0.1 (Mn2þ content), as shown in Fig. 5(d). Fig. 5(e) shows the relative intensity of the Y7(1-p-2q/3)Ce7pMn14q/ 3O6F9 (p ¼ 0.05, 0.1, q ¼ 0e0.1) phosphors when the excitation wavelength was 260 nm. When the Mn2þ content reached q ¼ 0.01 in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05) phosphors, the relative luminescent intensity was maximized. Once the maximum luminescent intensity reached the Mn2þ content corresponding to q ¼ 0.01, a further increase in the Mn2þ content in the phosphors led to a clear quenching of the intensity of the green emission. The Mn2þ green emission of Y7(1-p)Ce7pO6F9 (p ¼ 0.1) phosphor centered around 508 nm distinctly increased when the Mn2þ content corresponded to q ¼ 0.1. When the Mn2þ ions were gradually replaced by Y3þ ions, the smaller sites of Y(2)O4F3 and Y(3) O4F3 are preferentially expected to be occupied by Mn2þ ions. After the concentration of Mn2þ ions was increased, the activators possibly occupy 8-coordinated sites of Y(1)O4F4 and Y(4)O4F4 in the Y7O6F9 host lattice and the green emission was finally observed in shorter wavelengths owing to the weak crystal field effect. A multi-color-tunable emission from blue, white, yellow, and green regions was observed in the Y7O6F9 host lattice owing to the presence of Ce3þ to Mn2þ as a sensitizer and activator. As shown in Fig. 6, the chromaticity coordinates, x and y, were in accordance with the CIE values of the blue, white, yellow, and green yttrium oxyfluoride phosphors, corresponding to Y7(1-p-2q/3)Ce7pMn14q/ 3O6F9 with p ¼ 0.05, 0.1, q ¼ 0e0.1. In the insets of Fig. 6, photographs of the PL light emission are shown from the blue to orange

region in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors under 254 and 312 nm handheld lamps, and the CIE values are summarized along with the values obtained for the Ce3þeMn2þ co-doped yttrium oxyfluoride optical materials. The CIE coordinates near the blue, white, yellow, and green regions of the CIE diagram were observed to be x ¼ 0.175891 and y ¼ 0.102022 (A), x ¼ 0.260316 and y ¼ 0329757 (B), x ¼ 0.347798 and y ¼ 0.450856 (C), x ¼ 0.238905 and y ¼ 0.385863 (F) in the Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, q ¼ 0e0.1) phosphors, respectively.

4. Conclusions A single phase of Ce3þ, Mn2þ co-doped Y7O6F9 phosphors was successfully prepared using NH4F flux at 1000  C in a reducing environment. By doping a Ce3þ emitter in Y7O6F9 host lattice, efficient blue emission was achieved and the maximum luminescent intensity of Y7(1-p)Ce7pO6F9 phosphors was achieved when the Ce3þ content corresponded to p ¼ 0.05. The emission of Mn2þ ded transitions in the vernier host lattice was enhanced by co-doping a Ce3þ sensitizer through energy transfer from Ce3þ to Mn2þ and the resultant PL spectra were monitored. As the Ce3þ content was p ¼ 0.05 and 0.1 in Y7(1-p-2q/3)Ce7pMn14q/3O6F9 phosphors, noticeable quenching of the relative intensity of the Mn2þ content of q ¼ 0.01 and 0.05 occurred, respectively. When a ¼ 8, as deter/3 mined by the linear plots of IS/ISO of Ce3þ versus CaMn , the mechanism of the dipoleequadrupole interaction energyetransfer from Ce3þ to Mn2þ in Y7O6F9 phosphors was elucidated. The green and orange multi-color emission lights caused by the coordination environment of luminescent center of Mn2þ ions in Ce3þeMn2þ doped Y7O6F9 phosphors were concurrently exploited. The desired CIE values including emissions in the Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1, q ¼ 0e0.1) phosphors throughout the blue, white, green and orange regions of the spectra were achieved. The coedoping of Ce3þeMn2þ in the singleephase Y7O6F9 structure resulted in the multietunable emission lights, which would be effective for applications requiring NUV executable LEDs.

Fig. 6. The chromaticity coordinates with the CIE values of Y7(1-p-2q/3)Ce7pMn14q/3O6F9 (p ¼ 0.05, 0.1, q ¼ 0e0.1) phosphors and photographs of the PL light emission from blue, white, yellow, and green in the vernier phosphors under 254 and 312 nm handheld lamps (inset). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2015R1D1A1A01059655) and the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (2014H1C1A1066859). References [1] [2] [3] [4]

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